This invention relates to electronic amplifiers and more particularly to coupler-based amplifiers.
FIG. 1 is a circuit diagram of a conventional coupler-based amplifier 100 that includes two amplifier devices 102, 104. This circuit architecture is typically used when the output power to be delivered to the load ZL exceeds the capability of a single amplifier device. In the figure, the source is indicated as a voltage source and a series impedance ZS. When the 3-dB couplers 106, 108 are of quadrature type, e.g., field-coupled lines with an electrical length of one-quarter wavelength, mismatches between the individual amplifier devices 102, 104 and the couplers do not appear at the source and load connections. It will be noted that as shown, the secondary connections of the couplers are terminated in the characteristic impedance ZO. This architecture also provides some protection against amplifier device failure in that if one device breaks down, the amplifier still works but with reduced output power.
Coupler-based amplifiers are known in the art. U.S. Pat. No. 4,656,434 to Selin discloses a coupler-based power amplifier where load mismatch is compensated by feedback from the output coupler to the input coupler. U.S. Pat. No. 6,297,696 to Abdollahian et al. discloses a coupler-based power amplifier with dynamic load impedance matching based on measured reflected output power. U.S. Pat. No. 6,515,541 to Cheng et al. discloses a coupler-based power amplifier with impedance modification circuits coupled to the isolated ports of the couplers, in which the impedances are modified when one amplifier is turned off. U.S. Patent Application Publication No. 2002/0186079 to Kobayashi discloses a balanced amplifier with a coupler connected to the inputs of the amplifiers and matching networks connected to the outputs of the amplifiers.
A problem with conventional amplifiers such as those described above is that when the amplifier is connected to a varying load impedance ZL, the impedances ZL1, ZL2“seen” by each amplifier device diverge from the nominal value of ZL, e.g., 50 Ω. If the load is purely resistive, the impedances diverge in opposite directions with respect to both the real and imaginary parts. One amplifier device “sees” an impedance that is approximately equal to the load impedance (i.e., ZL1≈ZL), and the other amplifier device “sees” the impedance of a λ/4-transformed load impedance (i.e., ZL2≈ZO2/ZL). This in turn leads to diverging signal amplitudes Vout1, Vout2, respectively, from the amplifier devices 102, 104.